The present invention relates to an optical transmission system for high-bit-rate transmission of optical signals having a number of optical fiber link sections with, in each case, one optical fiber and one dispersion compensation unit, the dispersion compensation units having different absolute-magnitude compensations.
In optical transmission systems with high data throughput rates (thus, as in the case of optical transmission systems operating according to the WDM (Wavelength Division Multiplexing) principle as well as in the case of optical single-channel transmission systems), the chromatic dispersion occurring in the transmission of optical signals over the optical fiber and nonlinear effects such as self phase modulation (SPM) or cross phase modulation (XPM) cause distortions in the optical signals to be transmitted. In this connection, see Grau and Freude: “Optische Nachrichtentechik—Eine Einführung”, [“Optical Telecommunications Engineering—An introduction”], Springer-Verlag, 3rd Edition, 1991, pages 120 to 126.
Such distortions in the optical signal or data signal to be transmitted depend, inter alia, on the optical launch power of the optical signal. The distortions caused by the chromatic dispersion and the nonlinear effects produce a regeneration-free transmission range for an optical transmission system that is determined, in particular, by the signal-to-noise ratio required for the restoration of the optical signal at the end of an optical fiber link section. The transmission range that can be spanned without regeneration is to be understood here as the optical transmission link over which an optical data signal can be transmitted without the need to carry out a regeneration or “3 R regeneration” (electronic data regeneration with respect to the amplitude, edge and clock of an optically transferred, digital data signal or datastream).
In order to compensate such distortions in the optical data signal, suitable dispersion compensation units are provided or dispersion management is conducted in a fashion adapted to the optical transmission link in the case of the transmission of optical signals over optical standard single-mode fibers. The term dispersion management is to be understood here as a specific arrangement of dispersion compensation units along the optical transmission link (for example, at optical transmitters, at optical repeaters and/or at optical receivers), and as the determination of the suitable dispersion absolute-magnitude compensations for the different dispersion compensation units. Because of the transmission range that can be bridged without regeneration, optical transmission systems are assembled from a number of optical fiber link sections in which the fiber dispersion caused, in each case, in the optical fiber link section under consideration is virtually completely compensated, or partially overcompensated or undercompensated, with the aid of a dispersion compensation unit.
Such dispersion compensation units are configured, for example, as special optical fibers in the case of which the dispersion or fiber dispersion, in particular in the transmission wavelength region, assumes very high negative values owing to a special selection of the refractive index profile of the fiber core and in the surrounding cladding layers of the optical fiber. The dispersion contributions generated by the optical transmission fibers, such as a standard single-mode fiber, can be effectively compensated with the aid of the high negative dispersion values caused by the dispersion-compensating fiber. In addition, the maximum number of optical fiber link sections or the bridgeable range of the optical transmission system can be fixed via the eye pattern (eye opening) of the signal-to-noise ratio of the optical signal or data signal present at the output of the respective optical fiber link section. The minimum eye opening, required for the reconstruction of the optical data signal at the end of the optical fiber link section, of the eye pattern or of the signal-to-noise ratio required therefor, results in a maximum range for a regeneration-free transmission of an optical data signal.
Various dispersion management concepts are adopted for this purpose in optical transmission systems implemented to date, it being possible to carry out the optimum dispersion compensation of an optical transmission link by using optical fiber link sections that are precompensated and/or subsequently compensated or differently overcompensated or undercompensated. A spatially defined distance therefore can be bridged with a fixed number of fiber link sections as a function of the respective data rate, the data format and the fiber properties.
German patent application 19945143.5 discloses for this purpose a dispersion compensation scheme for an optical transmission system in the case of which optical signals are transmitted with data rates of around 10 Gbit/s over a fixed number of optical fiber link sections. In order to increase the transmission range of the optical transmission system, the absolute-magnitude compensations of the dispersion compensation unit at the end of each optical fiber link section are dimensioned in such a way that the remaining accumulated residual dispersion per optical fiber link section rises at least approximately uniformly by the same absolute-magnitude dispersion in each case. That is, the accumulated residual dispersion calculated or estimated for the entire optical transmission system is distributed virtually uniformly over the optical fiber link sections, and as a result each optical fiber link section is undercompensated by virtually the same absolute-magnitude compensation.
Furthermore, U.S. Pat. No. 5,629,795 discloses an optical transmission system that includes a number of optical fiber link sections with, in each case, one optical fiber and one dispersion-compensating medium. The optical transmission system is divided for this purpose into a multiplicity of optical fiber link sections. In each of these optical fiber link sections, with the exception of the last one, the dispersion-compensating media are used together with the respective fiber link section to compensate the accumulated wavelength dispersion completely or partially (undercompensation). The timing jitter caused in the optical transmission signal by the Gordon House effect is virtually completely eliminated by the described procedure. The optical signals transmitted in this process are transmitted in return-to-zero format with a transmission rate of approximately 20 Gbit/s. Such a dispersion management certainly leads to a reduction in the timing jitter caused by the Gordon House effect in the case of transmission bit rates of 20 Gbit/s, but it is impossible thereby to achieve any substantial improvement in range, in particular for high-bit-rate optical transmission systems with data transmission rates greater than 20 Gbit/s.
It is, therefore, an object of the present invention to configure an optical transmission system for high-bit-rate transmission of optical signals of the type mentioned at the beginning in such a way that the signal distortions caused by the fiber dispersion are reduced, and the transmission range that can be bridged without regeneration is increased.